The research of solar cells is an expected direction in renewable energy sources. Although the majority of the current commercialized products adopt silicon as the main material, organic solar cells attract attention by industry and academic community due to their simple process, low cost, light material, and flexibility.
Nonetheless, there are still many drawbacks in organic solar cells, for example, low conversion efficiency, low carrier mobility, and low device lifetime.
The active layer of organic solar cells has the functions of light-energy absorption, exciton dissociation, namely, electron-hole pairs, and electron and hole conduction. Because electrons and holes tend to encounter and recombine with each other in a single material, the efficiency of the organic solar cells having a single active layer is not high; they are mainly prepared using a double-layer structure.
In double active layer organic solar cells, the electron donors absorb light energy and become excitons. When the excitons transport to the interface between the electron donors and electron acceptors, they are dissociated into electrons and holes. Then, the electrons transport to the cathode layer in the electron acceptors; the holes transport to the anode layer, which is composed by indium tin oxides, in the electron donors.
Because electron donors and electron acceptors can conduct electrons and holes individually, recombination of electrons and holes is reduced substantially. Nonetheless, since the area of the interface of exciton dissociation is limited to the contact area between electron donors and electron acceptors, even the conversion efficiency of double-layer organic solar cells is higher than that of single-layer organic solar cells, the former are still not ideal in structure.
There is an improved structure named bulk heterojunction organic solar cells. According to the structure, electron donors and electron acceptors are both dissolved and mixed in organic solution and forming the active layer. After proper processes, phase separation occurs in electron donors and electron acceptors and thus forming transport channels for holes and electrons. In addition, thanks to phase separation, the spots for dissociating excitons on the contact area between electron donors and electron acceptors increase, thus increasing the photocurrent.
Currently, the electron donors normally used are Poly(3-hexylthiophene-2,5-diyl) (P3HT); the electron acceptors normally used are [6,6]-phenyl-C61-butyric acid methylester (PCBM). As the excitons are dissociated into electrons and holes at the interface between P3HT and PCBM, electrons will be conducted to the cathode while holes will be conducted to the anode. At this time, the holes generated near the anode can be conducted to the anode faster and becoming photocurrent. The holes farer to the anode need more time to be conducted to the anode. Besides, in the conduction path, recombination with holes is more. Moreover, the hole mobility of P3HT is lower than the electron mobility of PCBM. Thereby, there is greater resistance along the channel of conducting holes to the anode.